I don't think it's "common knowledge" at all, that Python/Ruby are "best" for this. They might arguably be "most popular" but "most popular" does not necessarily mean "best". It's entirely possible that there isn't even any such thing as "the best" language platform for this. There are a lot of variables.
Anyway, if you already know Java and have some familiarity with that world, I can't recommend Groovy and Grails highly enough. Grails has been a tremendously productive environment for me, FWIW.
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I don't think it's "common knowledge" at all, that Python/Ruby are "best" for this. They might arguably be "most popular" but "most popular" does not necessarily mean "best". It's entirely possible that there isn't even any such thing as "the best" language platform for this. There are a lot of variables.
Anyway, if you already know Java and have some familiarity with that world, I can't recommend Groovy and Grails highly enough. Grails has been a tremendously productive environment for me, FWIW.